1. Field of the Invention
Aspects of the present invention relate to an image forming apparatus, and more particularly, to an image forming apparatus which prevents a multi-sheet feed along a transportation path.
2. Description of the Related Art
Generally, an image forming apparatus, such as, for example, a printer, a copier, a fax machine, or a multifunctional apparatus includes a printing unit which prints an image onto a recording medium and a transportation path which transports the recording medium inside the printing unit. This recording medium may be various different types of media, such as a sheet of paper, a sheet of photographic printing paper, OHP film, etc., and will be referred to as paper hereinafter, but is not limited to paper. Sheets of paper should be moved one by one through the printing unit during printing operations, from the time the sheets enter the printing unit until the time the sheets are discharged from the printing unit to a discharge tray. If two or more sheets of paper are moved through the printing unit simultaneously (a situation referred to as a multi-sheet feed), the extra sheets may cause a paper jam in the image forming apparatus, or may cause a printing defect in the images printed on the sheets of paper, such as by printing images intended to be printed on different sheets of paper onto the same sheet of paper.
An image forming apparatus of the related art may include a multi-sheet feed preventing unit which prevents several sheets of paper from being moved through the printer at the same time. Examples of multi-sheet feed preventing units are described below.
A knock up plate is disposed in a sheet-feeding cassette which stores the paper that is eventually supplied to the printing unit. The paper is stacked on the upper side of the knock up plate, and a knock up spring elastically biases the lower side of the knock up plate in an upwards direction. Sheets of paper stacked on the upper side of the knock up plate are picked up individually by a pick up roller, and then are supplied to the printing unit by a feed roller.
One example of a multi-sheet feed preventing unit includes finger members which are disposed at both sides of the knock up plate, such that the pick up roller picks up the sheets of paper individually (one by one). The finger members apply a transportation obstructing force on the sheets of paper by pressing both corners of the sheets of paper stacked on the knock up plate. The frictional force between the pick up roller and a top sheet of paper stacked on the knock up plate is greater than the frictional force between the top sheet of paper and a next sheet of paper stacked under the top sheet of paper. Furthermore, the frictional force between the pick up roller and a top sheet of paper stacked on the knock up plate is greater than the transportation obstructing force of the finger members. Thus, the frictional force generated by the pick up roller picks up the top sheet of paper, while the frictional force between the finger members and the sheets of paper prevents extra sheets of paper from being picked up, and the sheets are picked up individually.
A second example of a multi-sheet feed preventing unit includes a friction pad disposed on the upper side of the knock up plate which frictionally prevents two sheets of paper from being picked up simultaneously by the pick-up roller, when only two sheets of paper are stacked on the upper side of the knock up plate. In this second example, the friction pad is disposed at a position facing the pick up roller housed inside the frame of an image forming apparatus. The frictional force between the pick up roller and a top sheet of paper stacked on the knock up plate is greater than the frictional force between the top sheet of paper and the next sheet of paper stacked under the top sheet of paper. Furthermore, the frictional force between the pick up roller and a top sheet of paper stacked on the knock up plate is greater than the frictional force acting on the friction pad and the paper. Thus, the sheets of paper are picked up individually.
As described above, a sheet of paper picked up by the pick up roller is supplied to the printing unit by the feed roller. The sheet of paper is supplied to the printing unit along the transportation path, and an image is printed onto the sheet of paper. The sheet of paper with the printed image is transported along the transportation path and then discharged outside the image forming apparatus to a discharge tray. On the transportation path are rollers positioned at the entrance and exit of the printing unit and a guide member, positioned between the rollers, which guides the sheet of paper along the transportation path. The rollers contact each other at a roller portion where the rollers face each other, forming a nip which nips a sheet of paper between the two rollers, and the rollers include a guide member portion which connects the rollers. The multi-sheet feed preventing unit may be disposed at a position adjacent to the pick up roller, or may be disposed at any position along the transportation path. Although the various multi-sheet feed preventing units described above can prevent multiple sheets of paper from moving through the transportation path simultaneously, sometimes these various multi-sheet feed preventing units fail to prevent multi-sheet feeds. Thus, an improved multi-sheet feed preventing unit is necessary.